The complement system is involved in defense against infections either by direct lysis of antibody-coated bacteria or by removal of bacteria and viruses by immune adherence and enhanced phagocytosis. The activation of either the classical or the alternative complement pathway converges on the cleavage of C3 into C3a and C3b fragments and the binding of C3b to membranes via its labile binding site. It has been suggested that human C3b fragment binds to receptive surfaces by cydrophobic interactions as well as by an ester-like bond. However, the mechanism of the reaction, and the groups involved are still unknown. We have shown previously that the enzymatic cleavage of C3 into C3a and C3b and also nondegrative inactivation of C3 (and C4) are accompanied by the concomitant appearance of a single sulfhydryl (SH) group. The main emphasis of the proposed research is on the mechanism of C3b interaction with cell surfaces. We propose to study: 1) the presence and identity of a putative chemically reactive group, which could participate in ester-like bond formation; 2) the location and microenvironment of the SH group present in C3d fragment, leading to the investigation of a possible catalytic role of the SH group in the binding of C3b to cell surfaces by e.g. transesterification; 3) conformational changes accompanying the enzymatic conversion of C3 and a nondegradative chemical inactivation of C3 by various physico-chemical methods, and 4) the spontaneous x-chain degradation of C3 (and C4) in the presence of denaturants in order to establish the specificity of cleavage site and to test the hypothesis that the cleavage is an autocatalytic reaction, a manifestation of an inherent property of native C3 and C4, respectively.